ERD plugin manual
This plugin enables SQLiteStudio to visualize database schema as entities (tables) and relations (foreign keys). It also allows to edit the schema in the same (visual) manner.
To open ERD editor for certain database, have a database (or any object inside that database) highlighted on the databases list. Then hit the ERD editor button on toolbar:
If you don't have button with this icon on your toolbar, check if you have the ERD plugin loaded into SQLiteStudio (go to Configuration window, then to Plugins section).